Flotation systems are used in several industries as a primary method of separating a desirable component from waste components. The mineral processing, oil sands and environmental engineering industries, for example, all have major applications for flotation. The problem associated with all flotation systems, as a cleaning process, is the tendency for some fraction of the waste components to be transported into the product stream. Several different forces inherent in the flotation process and in machine designs are responsible for this occurrence, e.g., entrainment and entrapment.
Flotation machines can have different features and designs depending on their application. The flotation machine design used to float combustible solids, i.e., coal, is typically a rectangular or square shaped cell that has impeller assembly, including an agitator and aerator. A commonly used flotation machine design for coal is shown in FIG. 1.
This conventional flotation cell is designed to maximize the contact of air with a coal slurry. The cell 1 has a impeller assembly 2 that includes a standpipe 3. The lower portion 4 of the impeller assembly 2 act to draw slurry, water and air through the impeller. Air enters through inlet 7 and is drawn down into the cell 1 for mixing with a feed slurry. The slurry is introduced into the cell 1 via inlet 8. The impeller assembly 2 has a disperser 5 that is used to disperse the air into minute bubbles. The hydrophobic coal particles and/or microagglomerates attach to the bubbles and are levitated to the top of the cell forming a froth 6. The froth 6 is removed by mechanical means, such a skimmer.
Another type of flotation machine design is directed to column flotation. A typical design is shown in FIG. 2. Column flotation has received much attention in the past five years. The process is based on the principle of counter-current flow of the impurity particles, i.e., the mineral matter, and coal particles.
Referring to FIG. 2, a flotation column 10 has a washing zone 11 and a collection zone 12. A feed inlet 13 introduces a coal slurry mixture into the column 10. Heavier mineral matter falls to the bottom of the column 10 due to gravity. Gas bubbles are formed by means 14. The coal particles attach to the gas bubbles and are floated to the top of column 10. A gentle spray of water from means 15 is used to wash the froth to liberate any entrained mineral matter.
Through the use of flotation, the sulphur and ash content of coal can be reduced, thereby improving its quality. However, due to similarities in surface chemistry characteristics, a small fraction of pyrite and certain other minerals will float together with the coal. As a result, accumulations of pyrite and other minerals in the collection zone and product stream of a flotation cell can be observed. Consequently, the separation efficiency for pyrite and certain other minerals will be limited.
The process disclosed in U.S. Pat. No. 4,966,608, incorporated by reference herein in its entirety, is capable of selectively forming microagglomerates of the combustible solids component of finely ground coal (d.sub.50 =150 .mu.m). In this process, pyritic sulfur and certain other mineral rich particles may be transported into the product stream during flotation due to hydrodynamic forces, i.e., entrainment, and by these particles being attached to the microagglomerates, i.e. entrapment. Pyrite particles are often difficult to remove from the product stream and are a source of sulphur in coal that cause increased emission of sulphur compounds into the atmosphere when the coal is burnt. This contributes to the occurrence of acid rain.
The purity of the recovered product can be improved, as taught by the present invention, by sprinkling a flotation froth with water to wash the impurities and other loosely held particles from the froth. When processing coal, conventional flotation machine designs provide no areas for washing and settling of pyrite and other ash forming particles. The present invention is directed to a hydraulic separator that can be used as a second stage separator to improve the quality of most flotation product streams.